In a CATV system, a head end subsystem, which typically includes a remote community antenna, gathers video signals from a variety of sources (e.g., satellite broadcasts, local and distant VHF and UHF stations, and the like) and combines these signals to form a composite CATV signal. The composite CATV signal comprises a wide band of frequencies, ranging from approximately 50 to 450 MHz, and is divided into over 30 6-MHz-wide narrow bands of frequencies, called channels, including channels 2-13 familiar to non-pay television viewers. The composite CATV signal is supplied from the head end subsystem to local subscribers, principally homeowners, who receive the signal via a coaxial cable. Once received, the desired channel must be separated from the composite CATV signal and then supplied to the subscriber's television.
A device for selecting one channel from the composite CATV signal is known in the art as a CATV converter and has been described, for example, in Ma U.S. Pat. No. 4,352,209. To minimize interference between channels, an ideal CATV converter must pass intact all components of the CATV signal within the narrow band of frequencies corresponding to the selected channel and totally reject all other components. Real CATV converters approximate this ideal behavior by stringing together electrical filters, called tunable resonant circuits. Each tunable resonant circuit is tuned to a particular frequency, called its resonant frequency, where the components at the resonant frequency, or within a narrow band of frequencies clustered about the resonant frequency, are passed and all other components are attenuated. The more tunable resonant circuits that are utilized, the more effectively out-of-band components are rejected. However, the number of tunable resonant circuits has an upper limit where, for example, the cost of adding one more tunable resonant circuit outweighs the benefit derived.
In accordance with this economic upper limit, the CATV converter described in the Ma '209 patent utilized four tunable resonant circuits for channel selection. Each tunable resonant circuit included a fixed inductive element and a variable capacitive element for tuning the circuit. However, variable capacitors respond erratically as their temperature varies, as they age, or as they undergo mechanical shock, such as by dropping. Consequently, the performance of the CATV converter might vary with temperature, time or the amount of handling it undergoes.